1. Field of the Invention
The present invention relates to a method and apparatus for transmitting frames in a Wireless Local Area Network (WLAN).
2. Description of the Related Art
In WLANs based on the IEEE 802.11 standard, a medium is occupied using a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocol.
In the CSMA/CA protocol, each station within a Base Service Set (BSS) senses a medium, and accesses the medium when other stations do not occupy the medium.
However, if the stations are apart from one another over a medium sensing range, one station cannot recognize whether another station occupies the medium.
In a WLAN, first, second, and third stations are in a service area of an AP. That is, the AP is in a sensing range of each station.
The second station can be in a sensing range of the first station while the third station is not. The first station and the third station can be in a sensing range of the second station.
That is, the first station can sense whether the second station occupies a wireless medium with the AP, but cannot sense whether the third station occupies the medium.
Similarly, the third station can sense whether the second station occupies the medium with the AP, but cannot sense whether the first station occupies the medium.
Accordingly, the first station and the third station can simultaneously attempt to occupy the wireless medium when the second station does not occupy the wireless medium.
A collision caused by simultaneous accesses of multiple stations to a wireless medium is called a “hidden node problem.”
In order to solve the “hidden node problem,” the IEEE 802.11 suggests a Request To Send/Clear To Send (RTS/CTS) mechanism.
According to the RTS/CTS mechanism, the station is forced to exchange a short frame (e.g., RTS frame, CTS frame, etc.) with the AP to block other stations from accessing the medium when the station transmits a frame longer than a set frame threshold (dot11RTSThreshold) value.
In the RTS/CTS mechanism, each station to transmit a frame longer than a threshold value first transmits an RTS frame to the AP and, upon receipt of the RTS frame, the AP broadcasts a CTS frame to the station in its service area.
In response to receiving the CTS frame, each station waits without occupying the wireless medium, and the station transmitting the RTS frame occupies the wireless medium to transmit a frame.
In the RTS/CTS mechanism, the threshold value can be arbitrarily set. The RTS/CTS mechanism can or cannot be used for all frames depending on the threshold value.
The RTS/CTS mechanism can partially solve the “hidden node problem.” However, when all stations transmit a frame longer than the threshold value, they are always required to process the RTS/CTS mechanism, thereby degrading use efficiency of wireless network resources and causing frame transmission delay by an RTS/CTS mechanism performance time.
For example, it is unnecessary for a station that can sense whether each station in the service area of the AP occupies the medium, like the second station, to perform the RTS/CTS mechanism in order to transmit a frame.
Accordingly, there is a need for a method capable of allowing each station in a WLAN to solve a “hidden node problem” through the RTS/CTS mechanism, maximizing use efficiency of a wireless network, and minimizing frame transmission delay.